A switchable color filter of the limited color type typically comprises a single electro-optic switching device that is positioned between color selective and neutral polarizing filters to selectively provide a light output of one of two primary colors. One type of electro-optic switching device used in such a color filter is a twisted nematic liquid crystal cell. A switching circuit connected to the twisted nematic cell delivers an electrical signal of selectable amplitude or frequency to rotate by either 0.degree. or 90.degree. the plane of polarization of light rays passing through it. Each of the two polarization directions provides a light output of one of the two primary colors. The colors of light that can be developed at the output of the color filter range along a line in color space between the two primary colors.
Another type of electro-optic switching device used in such a color filter is described in copending application of Philip J. Bos and Philip A. Johnson, Jr., Ser. No. 06/493,106, filed May 9, 1983, now U.S. Pat. No. 4,582,396 for "Field Sequential Color Display System." This patent application describes the use of a liquid crystal variable optical retarder as the electro-optic switching device. A switching circuit selectively commands the variable optical retarder to its field aligned "ON" state to provide essentially zero optical retardation for light rays of all colors and to its partly relaxed "OFF" state to provide half-wave optical retardation of light rays of the color to which the retarder is tuned. Each of the two optical retardation states provides a light output of one of the two primary colors. The colors of light developed at the output of the color filter also can range along a line in color space between the two primary colors.
Neither of the above-described switchable color filters is capable of providing with a single electro-optic switching device a light output of three colors that define a color gamut encompassing an area of color space. It is also known that a switchable color filter of the full color type provides a light output of three colors but requires at least two electro-optic switching devices to accomplish the task.